1. Technical Field
The present invention relates to a structure of a bush bearing.
2. Related Art
In a rack and pinion steering apparatus, a rack shaft with rack teeth, which meshes with teeth of a pinion, is movably supported via a bush bearing by a gear housing. Various synthetic resin-made bush bearings are proposed as the bush bearing. Typically, the synthetic resin-made bush bearing has a tightening margin with respect to the rack shaft, and movably supports the rack shaft.
When the bush bearing has a large tightening margin with respect to the supported rack shaft, the bush bearing can support the rack shaft so as to maintain a predetermined rigidity in a radial direction. However, when the rack shaft is tightened too much, the rack shaft has a high sliding frictional resistance, and thus is unlikely to slide. In contrast, when the bush bearing has a small tightening margin with respect to the supported rack shaft, the rack shaft has a low sliding frictional resistance, and has satisfactory moving characteristics. However, the rack shaft is likely to have a large center offset, or a gap or the like is likely to occur between the bush bearing and the rack shaft. In addition, the rigid support in the radial direction weakens, and when the rack shaft moves, rattle noise (gear rattle noise) is caused to occur between the teeth of the pinion and the rack teeth.
A gap between the synthetic resin-made bush bearing and the rack shaft is caused to occur by stress relief in association with creep deformation and thermal history of the synthetic resin. Accordingly, the rigid support in the radial direction weakens, and rattle noise (gear rattle noise) occurs. In particular, when radial contraction is caused to occur by the stress relief of the synthetic resin, the synthetic resin-made bush bearing has an increasing tightening margin with respect to the rack shaft, and the sliding frictional resistance becomes high. When a bush supports the entire circumference of the rack shaft, the rack teeth and the bush interfere with each other, and the bush is cut.
Patent Literature 1 (JP-A-2007-9962) discloses the bush that has a concave-shaped or a flat-shaped inside inner circumferential surface; an outside inner circumferential surface which is disposed on an outside in the radial direction farther than the inside inner circumferential surface; and a slit which is provided so as to correspond to the inside inner circumferential surface, and the bush bearing that is made of an endless annular-shaped elastic member installed in a circumferential groove of the bush. Since the bush bearing made of the endless annular-shaped elastic member is installed in the circumferential groove of the bush, the bush bearing can support the rack shaft so as to maintain predetermined rigidity in the radial direction and a direction in which the bush bearing departs from the teeth of the pinion, and the inside inner circumferential surface can prevent external disturbance from causing displacement of the bush bearing toward an opposite direction of the rack teeth of the rack shaft. The bush bearing has a low frictional resistance in a movement in an axial direction of the rack shaft, and can movably support the rack shaft. In addition, a gap is formed between the outside inner circumferential surface and an outer circumferential surface of the rack shaft, on which the rack teeth are formed. An impact of the stress relief in association with the creep deformation and the thermal history can be reduced.